JPH02274813A - Primary recrystallizing annealing method for forming oxide layer having excellent nitriding ability - Google Patents

Abstract

PURPOSE:To stably manufacture a grain oriented electrical steel sheet having oxide layer excellent in nitriding ability by specifying oxidizing degree in atmosphere in a primary recrystallizing annealing at the time of executing hot rolling, cold rolling, primary recrystallizing annealing, nitriding treatment and finishing annealing to a steel slab of the specific composition. CONSTITUTION:The slab containing 0.8 to 6.8wt.% Si, 0.008 to 0.048% acid soluble Al and the balance Fe and inevitable impurities is heated at about 1000 to 1270 deg.C, then the hot rolling is executed to it and the cold rollings are executed one time or >= Z times contg. intermediate annealing. Successively, the primary recrystallizing annealing is executed together with decarbonization and after executing the nitriding treatment, by applying anneal-parting agent containing MgO as the essential component, the finishing annealing for aiming secondary recrystallization and purifying is executed. Then, the oxidizing degree (P H2O/PH2) in the atmosphere at the time of the primary recrystallizing annealing is made in the range of 0.15 to 0.8. By this method, the grain-oriented electrical steel sheet, which uniformly and stably executes the nitrogen absorption through the surface, is obtd. and useful to iron core for electrical equipment.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides crystal grains with a Miller index of (110) <001
> Unidirectional electrical steel sheet with orientation or (100) <0
The present invention particularly relates to the primary recrystallization annealing method in the manufacturing process of so-called grain-oriented electrical steel sheets that are strongly oriented in a certain crystal orientation, such as bidirectional electrical steel sheets with a 01> orientation. These steel plates are used as soft magnetic materials as cores of electrical equipment.

(Prior Art) As mentioned above, a grain-oriented electrical steel sheet is a steel sheet with a thickness of 0.10 to 0.35 am that is composed of crystal grains with a certain orientation and usually contains 4.5% or less of Si. It is.

This steel sheet is required to have excitation characteristics and iron loss characteristics as magnetic properties, and for this purpose, it is important to align the orientation of the crystal grains to a high degree. This integration of crystal orientations is achieved using a catastrophic grain growth phenomenon called secondary recrystallization.

In order to control secondary recrystallization, it is essential to adjust the primary recrystallization structure before secondary recrystallization and to adjust the fine precipitates or grain boundary segregation type elements called inhibitors. This inhibitor has the function of suppressing the growth of general primary recrystallized grains in the primary recrystallized structure and selectively growing certain oriented grains.

Typical precipitates include M and P.

Littmann (Japanese Patent Publication No. 30-3651) and J.E. May.

D, Turnbull (↑rans, Met, soc
, AIMB 212 (1958) P769/781)
presented MnS, Taro and Sakakura (Special Publication No. 15644 of 1972) presented AjN, Konaka (Special Publication of Publication No. 51-13469) presented MnSe, and Komatsu et al. presented (M, 5i)N. There is.

On the other hand, as elements that segregate at grain boundaries, Saito (Journal of the Japan Institute of Metals 27 (1963) P186/195) describes Pb,
Sb, Nb.

Although ^g4else1s and the like have been proposed, industrially they are only used as auxiliary substances to precipitate-type inhibitors.

The conditions necessary for these precipitates to function as inhibitors are not necessarily clear, but they have been described in the calibration (Tetsu to Hagane 53 (1967) P1007/1023), Kurosui et al. year) P175
/181°44 (1980) P419/424) can be summarized as follows.

(1) Before the secondary recrystallization, - the presence of a sufficient amount of fine precipitates to suppress grain growth of the normal grains.

(2) The size of the precipitates is large to some extent, and thermal changes do not occur too rapidly during secondary recrystallization annealing.

(Problems to be Solved by the Invention) Currently, there are three typical manufacturing methods for unidirectional electrical steel sheets that are industrially produced.

The first technique is a two-step cold rolling process using MnS, which was disclosed in Japanese Patent Publication No. 3651-1973 by M.P. AI shown in Publication No. 15644
This is based on a process in which the final cold rolling rate using N + MnS is strongly reduced to 80% or more.
(or Mn5e) +Sb by a two-time cold rolling process.

In order to ensure the amount of precipitates and to satisfy the requirements for making them finer, the basic technology of all of these techniques is to create an inhibitor by heating the slab at a high temperature before hot rolling.

High-temperature slab heating has the following problems.

1) A high-temperature slab heating furnace exclusively for grain-oriented electrical steel sheets is required.

2) The energy consumption rate of the heating furnace is high.

3) Oxidation of the slab surface progresses and molten material called slag is generated, which has a negative impact on operations.

In order to solve these problems and realize low-temperature slab heating, an inhibitor manufacturing technology that does not rely on high-temperature slab heating is required.

Some of the inventors of the present invention disclosed a manufacturing method of forming an inhibitor by nitriding treatment in Japanese Patent Publication No. 45285/1985 (unidirectional electrical steel sheet) and Japanese Patent Application No. 297825/1988 (bidirectional electrical steel sheet). is presenting.

What is important in this process is to uniformly precipitate and disperse the inhibitor by nitriding.

On an industrial scale, if there is non-uniform nitriding in the longitudinal and width directions within the coil, a corresponding problem arises in that the magnetic properties become non-uniform.

The rate-determining step in nitriding is the reaction on the surface, and in order to perform nitriding uniformly and stably, it is necessary to control the oxide layer formed on the surface during the next crystal annealing.

The present invention is characterized by -oxidation degree (PH20/PM
The present invention provides a method for creating an oxide layer with excellent nitriding ability by specifying .

(Means for Solving the Problems) The gist of the present invention is that Si: 0.8 to 0.8% by weight.
6.8%, acid soluble IV: o, 008-0.048%.

A slab consisting of the remainder Fe and unavoidable impurities is hot-rolled, annealed if necessary, cold-rolled to the final thickness, then primary recrystallization annealed, annealing separator applied, and finished. In a method for producing grain-oriented electrical steel sheets that includes an annealing process, nitriding is performed between after primary recrystallization annealing and before the start of secondary recrystallization in finish annealing to form an inhibitor necessary for secondary recrystallization. In order to perform nitriding uniformly and stably, the degree of oxidation (PH!O/P Hz) of the atmosphere during annealing is specified at 0.15 to 0.80, and an oxide layer with excellent nitriding ability is formed. The primary recrystallization annealing method is characterized by the production of

This will be explained in detail below.

The present inventors have determined that Si: 3.3%, acid-soluble Aj: 0.0
A hot-rolled sheet containing 27%, N: o, 0.08%, Mn: 0.14%, and the balance consisting of Fe and unavoidable impurities was heated to 110%.
After annealing at 0'C for 2 minutes, it is cold rolled to 0.20
The final plate thickness was M. This material has an oxidation degree (PH10/
Next crystal annealing was performed while changing Plh) in the range of 0.02 to 1.0.

Thereafter, an annealing separator containing MgO as a main component was applied, and final annealing was performed. Finish annealing is N, 25χ + Ht75χ
The temperature was raised to 1200°C in an atmosphere of

Figure 1 shows the nitrogen increase amount at 850°C, where the nitrogen amount is maximum, and the oxidation degree (PH1O/P) during primary recrystallization annealing. ).

From FIG. 1, nitriding is stable in the range of oxidation degree of 0.15 to 0.80, particularly preferably 0.25 to 0.70,
Correspondingly, the magnetic flux density (B-value) shown in FIG. 2 also becomes higher when the amount of nitrogen enrichment is large.

Although the theoretical basis for the large change in nitriding ability depending on the oxidation degree of the next crystal annealing is not completely clear,
The inventors conducted an investigation using analysis methods such as infrared spectroscopy and GDS, and found that the problem may be due to the non-uniform structure of the outermost layer of silica (Sing) and Firelite (Pe!5i04). thinking.

That is, as shown in FIG. 3, which shows an equilibrium state diagram of oxides, this limited range coincides with the formation region of habofireite. However, in reality, the outermost layer is mainly composed of Firelite when the oxidation degree is higher than 0.25;
In the lower part, silica is the main component, resulting in a heterogeneous structure that has not substantially reached an equilibrium state.

The reason why the nitriding ability changes discontinuously at an oxidation degree of 0.25 is considered to be due to such a qualitative change.

Further, the reason why the nitriding ability deteriorated rapidly below the oxidation degree of 0.15 is thought to be due to the formation of tight silica since the concentration of silica was particularly significant in the outermost layer.

In this way, the degree of oxidation (P HzO/
By limiting P Hl), the structure of the surface oxidation layer can be controlled, and the nitriding treatment performed from the next crystal annealing to before the secondary recrystallization during final annealing can be performed uniformly and stably. It becomes like this.

Next, embodiments of the present invention will be described.

In the present invention, the components contained in the slab include Si: 0.8 to 6.8% and acid-soluble Aj: 0.00% by weight.
8 to 0.048%, the balance being Fe and unavoidable impurities, and these are essential components, and other than that there are no particular limitations.

St increases electrical resistance and improves properties by lowering iron loss, but if it exceeds 4.8%, cold rolling becomes impossible. Furthermore, if it exceeds 6.8%, cracks are likely to occur even during warm rolling (and rolling becomes impossible), while if the content is lowered, α → γ transformation occurs during final annealing, destroying the crystal orientation. Therefore, the limited range is set to 0.8% or more, which does not cause α→T transformation at 950''C, which is considered to be the lower limit of the secondary recrystallization temperature.

Acid-soluble M is bonded to N to form AjN or (N).

5t) N and functions as an inhibitor. Especially-
When considering nitriding after standard crystal annealing, it is effective to allow M to exist as free M. The limited range is 0.008% to 0.048% where the magnetic flux density becomes high.

In addition, Mn, S, and Se are inhibitor constituent elements.

B4i, Nb, Sn, Ti, etc. can also be added.

If the heating temperature of this slab is less than 1000°C, it will be difficult to maintain the shape of the steel plate, and if it exceeds 127°C, problems such as slag mentioned above will occur.
270°C is desirable. The heated slab is subsequently hot rolled.

The hot-rolled sheet is annealed in a temperature range of 750 to 1200°C for 30 seconds to 30 minutes, if necessary.

Next, cold rolling is performed once or twice or more with intermediate annealing in order to obtain a predetermined final plate thickness and texture.

For unidirectional electrical steel sheets, basically
The final cold rolling should be 80% or more as shown in Japanese Patent Publication No. 15644, and for bidirectional electrical steel sheets, the method shown in Japanese Patent Publication No. 35-2657 or Japanese Patent Publication No. 38-8218 Cross cold rolling with reductions of 40-80% and 30-70% is applied. Thereafter, if carbon is contained in the steel, it is also subjected to decarburization and crystalline annealing.

Here, the gist of the present invention is to limit the degree of oxidation during annealing to 0.15 to 0.80, preferably 0.25 to 0.70, and to control the structure of the surface oxidation layer.

After applying an annealing separator containing MgO as a main component to the material thus obtained, final annealing is performed for the purpose of secondary recrystallization and purification.

There are no limitations on the nitriding treatment performed after the final crystal annealing and before the start of secondary recrystallization, including the conventional method of increasing the nitrogen partial pressure during the final annealing, and an atmosphere with nitriding ability such as ammonia. There are a method using gas, a method of adding a metal nitride having nitriding ability such as manganese nitride, chromium nitride, etc. to the annealing separator.

(Example) Example 1 In weight %, Si: 3.3%, acid soluble IV: 0.0
25%.

N: O, OO8%, Mn: 0.14%, S:
0.007%.

A slab containing 0.05% C and the remainder Pa and unavoidable impurities was heated to 1150°C and then hot rolled to form a hot rolled sheet with a thickness of 1.8 cm. This hot rolled plate is 1100
After annealing at ℃ for 2 minutes, 63 mm in the same direction as the hot rolling direction.
% cold rolling was performed, and further cold rolling was performed applying a rolling reduction of 55% in the direction intersecting the cold rolling direction, resulting in a final plate thickness of 0.30 m. The cold-rolled sheets thus obtained were subjected to primary recrystallization annealing at 810° C., which also served as decarburization, with varying degrees of oxidation.

Then, after applying MgO as an annealing separator, Nt:
The temperature was raised to 1200°C at a heating rate of 15°C/hr in an atmosphere of 25χ + Ht75χ, and then heated at 1200°C for 20
Time) Purification was performed in an atmosphere of It: 100χ. Table 1 shows the amount of nitrogen added during final annealing at 850°C and the magnetic properties of the obtained product.

Table 1 6 followed by NH.

The nitriding treatment was performed in a nitrogen atmosphere containing 3% of

- The relationship between the degree of oxidation and the amount of nitrogen enrichment during standard crystal annealing is as shown in Table 2.

Table 2 Example 2 In weight %, Sl: 3.2%, acid-soluble AIl: 0.0
27%, N: 0.007%, Mn: 0.13%, S
Contains 0.007% of i, 0.05% of C, and the balance is Pe.
After heating the slab containing unavoidable impurities to 1150° C., it was hot-rolled into a hot-rolled plate having a thickness of 1.8 mm. The hot rolled sheet was annealed at 1120° C. for 2 minutes, then at 900° C. for 2 minutes, and then cold rolled to a final sheet thickness of 0.20 mm. This cold-rolled sheet was decarburized at 8.30℃ by changing the oxidation degree.Example 3 The same cold-rolled sheet as in Example 2 was decarburized at 8.30℃.
Adjust the dew point so that it is constant, and (a) Nt25χ÷
H275χ, (b) s寞50χ+■850χ+ (c
) Nt7sχ+Hz25X in three types of atmospheric gases 83
Annealing was performed at 0°C. Next, nitriding treatment was performed in a nitrogen atmosphere containing 3% NHs.

As shown in Table 3, the amount of nitrogen increase is determined by the degree of oxidation, regardless of the atmospheric gas composition.

table After applying an annealing separator for 20 minutes, final annealing was performed.

Final annealing at 15°C in an atmosphere of N225χ + H875χ
The temperature was raised to 1200°C at 1200°C/hr, and purification was carried out at 1200°C for 20 hours at Hz 100χ. Table 4 shows the amount of nitrogen increase during final annealing at 850°C and the magnetic properties of the obtained product.

Table 4 Example 4 In weight %, Si: 3.2%, acid-soluble Al: 0.0
27%.

N: O, OO3%, Mn: 0.14%, S:
0.007%.

A slab containing 0.05% C and the balance consisting of Pe and unavoidable impurities was heated to 1150°C and then hot rolled to form a hot rolled sheet with a thickness of 1.8 am. This hot rolled plate is 1
After annealing at 100°C for 2 minutes and 900°C for 2 minutes, it was cold rolled to a final thickness of 0.20 wm.

This cold-rolled sheet was subjected to primary recrystallization annealing at 830° C., which also served as decarburization, with varying degrees of oxidation. Next, for the purpose of nitriding, the main material is MgO to which 5% of ferromanganese nitride is added. In the manufacturing method of grain-oriented electrical steel sheets, which focuses on the production of grain-oriented electrical steel sheets, the surface oxidation layer, which is the rate-limiting factor for nitriding, can be controlled, and nitrogen absorption through the surface can be performed uniformly and stably. Its industrial effects are enormous in stabilizing.

[Brief explanation of drawings]

Figure 1 shows the oxidation degree during primary recrystallization annealing and 85 during final annealing.
Figure 2 shows the amount of nitrogen enriched at 0°C, Figure 2 shows the degree of oxidation during primary recrystallization annealing and the magnetization characteristics of the product, Figure 3 shows the amount of nitrogen enriched at 0°C.
, FetSiO4,5iOt. 0.5 → Oxidation/1 (nθ/~2) θ5 Meθ Oxidation degree (P#Z(:l/PHi)

Claims (1)

[Claims] Si: 0.8 to 6.8% by weight, acid-soluble Al: 0.
A slab consisting of 0.008 to 0.048%, balance Fe and unavoidable impurities is processed through the steps of hot rolling, cold rolling, primary recrystallization annealing, application of an annealing separator, and final annealing, especially after primary recrystallization annealing. In a method for producing grain-oriented electrical steel sheets in which nitriding treatment is performed between the beginning and the start of secondary recrystallization during finish annealing, the oxidation degree (PH_2O/PH_2) of the atmosphere during annealing is set to 0.
．． A primary recrystallization annealing method for producing an oxide layer with excellent nitriding ability, characterized in that the nitriding ability is in the range of 15 to 0.8.